Cellulose esters of fatty acids and process of making the same



Patented May 8, 1928.

- UNITED STATES PATENT oFFicE.

HANS .T. CLARKE AND CARL J. MALM, ROCHESTER, NEW YORK, ASSIGNORS TOEASTMAN KODAK COMPANY, OF ROCHE YORK.

STER, NEW YORK, A CORPORATION OF NEW CELLULOSE ESTERS OF FATTY ACIDS ANDPROCESS MAKING THE SAME.

No Drawing. Application filed July 12,

This invention relates to cellulose esters of fatty acids and processesof making the same.

One object of the invention is to provide fatty acid esters of cellulosein which approximately one molecular proportion of acid (especially onehaving more than one carbon atom) is combined with an amount ofcellulose corresponding to twenty-four carbon atoms. Another object isto provide an inexpensive, stable, fatty acid ester of cellulose whichis very susceptible to further esterification. Still another object isto pro vide such an ester in a purified condition. A

further object is to provide an inexpensive and simple process forproducing such esters. Another object is to provide a process forproducing such esters in which no anhydrid is used, the fatty acid beingthe sole acylating agent. A still further object is to provide a processof makin fatty acid cellulose esters in one stage of W ichesterification is carried on without the aid of afatty acid .anhydrid,and in a subsequent stage of which further esterification is effectedwith the aid of such an anhydrid but in only economical amounts. Otherobjects will hereinafter appear. We have found that'hitherto undescribedfatty acid esters of cellulose can be prepared in which approximatelyone molecular proportion of-acid, containing less than eight carbonatoms, 1s comblned with anamount J of cellulose corresponding totwenty-four cellulose from which they have been carbon atoms. Aside fromthe scientific bearing of this fact upon the theory of the structure ofthe cellulose molecule, "these esters have properties of technicalimportance. They are relatively inexpensive, can' be obtained in asufficiently pure state and are very susceptible to furtheresterification.

Moreover, those derived from fatty acids having more than one and lessthan eight carbon atoms are very stable, as indicated, for example, bytheir resistance to hydrolysis under prolonged treatment with boilingwater. As a rule, the new compounds are superficially indistinguishablefrom the repared; for example, esters prepared rom cotton cellulosestill have the appearance of cotton fibers. w

We have likewise found that the process 1926. Serial No. 122,028.

of producing these esters is very simple and inexpensive, and can becarried out wlthout the aid of anhydrids of the fatty acids,

which are relatively expensive. It is merely necessary to heat thecellulose with the tion, with the acid as the sole acylating agent, tothe point where the above mentioned ester is obtained, usefulpracticalre 7 sults are effected by esterifying to the point where at lea'st4% ofacyl group is present in the compound:

We have also found that when th ese one in twenty-four esters arefurther esterified,

say by using anhydrids or acyl chlorides in the ways previouslydescribed for the aoylation of cellulose itself, technically usefuleconomies in anhydrid or acyl chloride can be obtained. Savings of 10%to 25% of the amount of anhydrid used have been noted in, this work. i

Broadly speaking, our invention is concerned with estersof the fattyacids having more than one and less than eight carbon atoms, such asacetic, propionic, butyric, valeric, caproic, heptylic, etc. But sincethe rate of esterification falls off rapidly upon ascending the abovecited homologous series, we have found that economy of time is obtainedbest when using'fatty acids having more than one and less than eightcarbon atoms. Of course, acetic acid, propionic acid and butyric acidare preferred, because of their relatively greater commercialavailability and their greater speed of reaction.

Our process can utilize cellulose from any of the sources customarilyused in the manufacture of esters, such, for instance, as sulfite woodpulp (preferably bleached), cotton,

surgical cotton wool, tissue paper especially prepared from cotton, etc.It is an advantage that our process can utilize relatively impurercellulose than is often employed in essential that it be employed,because of the whitening tendency of our process when applied to theunbleached or partially bleached pulp. It will be seen that the abovelist relates to cellulose and does not include hydrocellulose orhydracellulose.

The process may be conducted over a considerable temperature range. Wehave found that the production of one in twenty-four esters takes placerapidly with the lower members of the fatty acid series at 117 C, butthe working temperatures can range up to the point where the celluloseor the prodnets suffer from degradation. Ve find it inadvisable, as ageneral proposition, to use temperatures above 17 0 C. Within theworking range the temperature will vary with the particular acid ormixture ofacids which is employed and with the pressure under which theprocess is carried out. For instance, Working at atmospheric pressureand employing acids which boil below 170 C. at such pressure, we find ituseful to conduct the process by boiling the cellulose in the acid,preferably in a considerable excess of acid over the amount which willenter into the ester. But the time can be very considerably shortened byusing a superatmospheric pressure, provided the correspondingtemperature does not reach the point where the product is degraded.

While we have found it practical to carry out the process by heating thecellulose in the acid or mixtures of acids, as the sole treating bath,nevertheless, useful amounts of the acyl group may be introduced, sayWell above 4%, for example, when the acids are mixed with non-acylatingliquids, such as chloroform, carbon tetrachloride or benzene.

The amount of water present during the reaction is important because ithas a depressing effect upon the total percentage of acyl group whichcan be introduced. Taking into account both the initial water in thereagents and the water formed during the esterification, it should neverbe allowed to reach the point where it prevents the introduction of morethan 4% of the acyl group. We prefer, in fact, to maintain theconditions during the reaction such that water. tends to be remoyed ordiminished in quantity rather than increased in amount. The use-of areflux condenser, which will condense and. return to the bath the vaporsof the fattv acids without condensing the water vapor, is one way ofeffecting this result. Such a reflux condenseraction is not alwaysessential, especially when working with the lower members of the aboverecited series, provided the acid and cellulose are sufficiently low inmoisture. With the higher members of the series it is best to eliminatethe water during the reaction.

We shall now describe various embodiments of our process by way ofillustration, but it will be understood that the nvent on is not limitedto such details except as indieated in the appended claims.

As the boiling continues, in this example,. the esterification graduallytakes place and eventually the percentage ofacetyl group in the esterreaches the point where it is not materially increased by furtherboiling, say about 6%. This time varies somewhat with the type andcondition of cellulose em ployed, representative times, for example,being 160 to 200 hours. But a distinctly useful product is obtainedafter a much shorter time, say about 48 hours, for instance. At thattime the amount of acetyl group is very considerably above 1% and theester is eminently suited for rapid and ecomical acetylation in whichacetic anhydrid is used. As indicated above, the maximum amount ofacetyl group, which can be introduced by prolonged boiling, issubstantially one molecular proportion of the fatty acid combined withan amount of cellulose corresponding to twenty-four carbon atoms. It isconvenient to note that the percentage of acyl group in a given estercorresponds to a still higher percentage of combined acid, the relationbeing in the proportion of the molecular weight of the acyl group to themolecular weight of the acid. For example, 4% of acetyl group means5.58% of combined acetic acid.

,After the esterification has reached the desired point, the excess ofacid is removed if the ester is to be further esterified by treatmentwith an anhydrid and a catalyst.

Enough of the acid can be left with the one in twenty-four ester toproperly function as a diluent or solvent along with the anhydrid andcatalyst, as is customary in the acetylation of plain cellulose. Butwhere the lower ester is desired by itself. the excess of acid iscompletely wrung and washed out of it and the latter then dried. Acetylcellulose, thus obtained, has substantially the superficial appearanceof the cellulose from which it was prepared,'in other words, 1t lookslike cotton when 1t is prepared from cotton fibers, as in the aboveexample. Moreover, such an ester is remarkably stable and withstandsrather prolonged boilingin w'at'er wi thoutloss of the acetyl group. Ofcourse, in the above and in the acid, the latter may contain up to aboutof water. But in this case it requires a very much longer time beforethe proportion of acetyl group rises above 4%. It' is, therefore, highlypreferable to keep the water content lower than 10% and preferably below1%. v

In a further example, one part of cellulose, say cotton fibers of theusual purified form, is boiled under atmospheric pressure with. 30 partsby weight of propionic acid, provision being made for the removal of thewater during the boiling, say by a suitable reflux condenser. In generalthe percent- 'e of propionyl group rises to about 6% fter 48 hours andreaches a maximum of about 8% after between 100 and 200 hours boiling Inanother example one part by weight of cellulose, say cotton fiberssomewhat puri fied, is boiled at atmospheric pressure with 30 parts byweight of butyric acid. After 48 hours boiling the product contains inthe neighborhood of 5% of butyryl group and isuitable kind 'of celluloseis heated with 30;

after 200 hours has well above 10% of this ac 1 group present.

n another example one part of cellulose,

such as purified cotton fibers, say surgical cotton wool, is heated with30 parts by weight of heptylic acid, the materials during the reactionbeing kept between the temperatures of 145 and 165 C. This is below theboilin point of the acid, but'a notable esteri cation, nevertheless,takes place. By observing these temperature recautions the tendency todegrade the pro uct is lessened.

In all of the above examples the opera tions are conducted atatmospheric pressure. The reaction may be considerably hastened bycarrying out the action under pressure, say in a suitable acid-resistingautoclave. For example, one part of any part of acetic acid of 99 to100%- strength at a temperature of 140 to 160 (1, and under acorresponding superatmospheric pressure. Under such circumstances up.-

wards of 5% of 'acetyl'group will be found.

combined in about 2 hours,thus enormously shortening the time ofthe'operation.

The-esterification may beconducted with the acid in'the vapor phase. Forexample, cotton fibers, may be placed in a glass tube, heated by anysuitable means, such as an oil b'ath, and vapor of acetic acid conductedthrough the tube. It is convenient to operate at atmospheric pressurewith the It will be noted that no catalysts are used 7 in any of thehereinabove described examples; and consequently 'there can benodegradation from that source, even upon prolonged heating.

It is a remarkable fact that all of the esters of lowesterification'produoed in the above examples are especially susceptibleto further esterification, say by the methods customarily employed inesterifying cellulose itself. Moreover, the acyl group, which is furtherintroduced by the conventional methods, employing anhydrids and catalysts, need not be the same acyl grou as that originally present in thelower or one in twenty-four ester. Thus, for example, the cellulosepropionate, described above, may be acetylated and the final productwill contain both propionyl and acetyl groups. Obviously a widecombination of mixed esters may be obtained in this manner.

It is a characteristic of our new esters that further esterification canbe carried out with in acetic anhydri'd ranging between 10 and 25%.Because of the susceptibility of our low esters to furtheresterification, the latter! operation can beconducted with mildercatalysts, such as zinc chloride, thus avoiding any danger ofdegradation of the product through careless or incorrect use of sulfuricacid as the condensing agent. As these methodsof esterification withanhyrid and a weak catalyst are well known to those skilled in the art,it is unnecessary to describe them in detail here as they can be used intheir original form but with a useful lowering of the amount ofaceticanhydrid.

Having thus described our invention,

what we claim as new and desire to secure 2. Substantially unhydrolyzedcellulose acetate having the appearance of cellulose and being stable toboiling water at atmospheric pressure, the proportion of the acetylgroup therein being between 4.% and 6.5%.

3, In the process of [making a cellulose ester of a fatty acid, heatingtogether cellulose and a fatty acid having more than one and less thaneight carbon atoms at a temperature-between 117 and 17 0 C. inclusiveuntil substantially one molecular proportion of said fatty acid combineswith an amount of cellulose corresponding to twenty-four carbon atoms,said acid being the sole acylating agent.

4. In the process of making a cellulose ester of a fatty acid, heatingtogether substantially unhydrolyzed cellulose and a fatty acid havingmore than one and less than eight carbon atoms, said acid being the soleacylating agent, the temperature during the reaction being kept belowthe decomposition points of the ingredients and product, and theproportion of water in the reaction mass permitting the acyl group inthe ester to reach at least 4.72,.

5. In the process of making a cellulose ester of a fatty acid, heatingtogether cellulose and an excess of a fatty acid having more than oneand less than eight carbon atoms, said acid being the sole acylatingagent, the reaction being conduct-ed at a pressure above atmospheric butthe temperature being maintained below the decomposition points of theingredients and product.

6. In the process of making cellulose acetate, heating cellulose in theabsence of a catalyst in an excessof acetic acid, the latter being thesole acetylating agent, until the acetyl group in the ester reaches atleast 4.%.

7. In the process of makin a fatty acid cellulose ester, partiallyesterif ying cellulose with a fatty acid having more than one and lessthan eight carbon atoms in theabsence both of an anhydrid and acatalyst, and thereafter esterifying it. further [with the aid of ananhydrid of a fatty acid.

8. In the process of making a fatty acid cellulose ester, heatingcellulose, together With a fatty acid having more than one and less thaneight carbon atoms until substantially one molecular'proportion of saidfatty acid combines with an amount ofcellulose corresponding totwenty-four carbon atoms, said acid being the sole acylatin agent, andthereafter further esterifying 1t with the aid of an anhydrid of a fattyacid.

Signed at Rochester, New York, this 21st day of June 1926.

HANS r. CLARKE. CARL .i. MALM.

